In writing and reading specific information on an optical disc, the laser beam that is incident on the optical disc must track the optical disc track precisely. This tracking is performed through controlling a tracking actuator using a tracking error signal that is obtained through illuminating the laser beam onto the optical disc.
Moreover, the laser beam that is incident onto the optical disc must be focused precisely into a focal point on the reflective surface of the optical disc, requiring control of the distance between the optical disc and the object lens that focuses the laser beam. To do this, a focusing error signal is produced and this focusing error signal is used in controlling a focus actuator.
The principle behind the three-laser beam method will be explained briefly next. As illustrated in FIG. 9, a main laser beam (A, B, C, and D) is positioned in the center, and to sub-laser beams (E and F) are positioned on either side thereof. The offset of the two sub-laser beams forward and back is to prevent loss of the detecting signal. In FIGS. 9(a), (b) and (c), the (b) position is where the spot of the main laser beam is directly over the track, showing the best state. In this state, the sub-laser beams E and F are both following so as to be slightly on the track, where the latter laser beam is on the so-called mirror surface wherein there are no pits, where the light of this part is reflected from the optical disc to arrive at the detector (not shown). The reflected beam signal from the sub-laser beam E and the reflected beam signal from the sub-laser beam F are inputted from the detector into a differential amplifier, illustrated in FIG. 10, and, in this case, the signal from the differential amplifier (the sub-tracking error signal) is zero.
Moreover, when the position of the laser beam deviates slightly from this, so that the laser beam position goes to that which is illustrated in (a) or (c) of FIG. 9, a signal that is the difference between the sub-laser beam E and the sub-laser beam F is outputted from the differential amplifier. In the case of (a), this is a positive output, and in the case of (c), this is a negative output, producing a polarized sub-tracking error signal. That is, this produces information regarding the side of the track to which there is misalignment, and information regarding the magnitude of the misalignment of the laser beam.
The circuit for producing the tracking error signal will be explained next. Here, the explanation will be for the case of the three-laser beam system, as illustrated in FIG. 9, wherein a main laser beam (A, B, C, and D) is incident on the track of the optical disc and respective sub-laser beams (E, and F) are incident on the mirror regions on either side of the track. The reflected beam of the main laser beam (A, B, C, and D), and of the sub-laser beams (E and F) are incident through optical pickups into detectors that convert the incident reflected beams into electric currents, and electric signals are outputted from the detectors.
At this time, the reflected beam of the main laser beam (A, B, C, and D), as illustrated in FIG. 9, is detected by a detector wherein a single spot is divided into four regions. The output signals from the detector are defined as A1, B1, C1, and D1, corresponding to the main laser beam. Moreover, the reflected beams from the sub-laser beams (E and F) are detected by the respective detectors. The output signals corresponding to the sub-beams from the detectors are defined as E1 and F1.
The production of the tracking error signal by the circuit illustrated in FIG. 11 will be explained using these signals. The circuit in FIG. 11 is a tracking error signal generating circuit 110 that is structured from a main tracking error signal generating circuit, a sub-tracking error signal generating circuit, and other circuitry. The tracking error signal generating circuit 110 has amplifiers 120 to 123 for amplifying the signals from the detectors, and low-pass filters 130 to 133 for removing the noise components.
Moreover, the tracking error signal generating circuit 110 comprises A/D converters 140 to 143 for converting into digital signals the analog signals F1 and E1, and (A1+D1) and (B1+C1), after the amplification process by the amplifiers 120 to 123 and after the filter process, and amplifiers 150 to 153, for adjusting the balance of the outputs of the signals F1, E1, (A1+D1), and (B1+C1) after the A/D conversions.
The difference between the output from the amplifier 150 and the output from the amplifier 151 is taken and inputted into a gain controller (AGC) 160. Similarly, the difference between the output of the amplifier 152 and the output of the amplifier 153 is taken and inputted into an AGC 161. Moreover, the tracking error signal generating circuit 110 is provided with low-pass filters 170 and 171 for removing the respective noises included in the outputs of the gain controllers (AGCs) 160 and 161. The output of the low-pass filter 170, from which the noise has been removed, is the sub-tracking error signal, and the output from the low-pass filter 171 is the main tracking error signal. The difference between the sub-tracking error signal and the main tracking error signal passes through an attenuator 180 and a low-pass filter 190 that adjust the signal level, to produce the tracking error signal.
Moreover, Patent Document 1 describes the generation of a stabilized tracking error signal wherein saturation of the output of the sub-tracking error signal due to eccentricity of the optical disc, or the like, is prevented.
Furthermore, Patent Document 2 describes a technology for performing tracking by detecting the magnitude of eccentricity of the optical disc and selecting a tracking error signal produced through the DPD method (Differential Phase Detection method) if the magnitude of eccentricity is greater than a value that is set in advance, or using a tracking error signal produced through the DPP method (Differential Push-Pull method) if the magnitude of eccentricity is smaller than the value that is set in advance.